The present invention relates to lasers, and more particularly to an electromagnetically tunable broadband laser having no moving mechanical parts.
The laser is a device that generates coherent electromagnetic radiation in, or near, the visible part of the spectrum. The term "laser" refers to Light Amplification by Stimulated Emission of Radiation. In very simple terms, a laser operates by applying energy to a suitable material, termed the "laser medium", such as a ruby rod or semiconductor. This energy pumps electrons in the material to higher energy states. These "pumped up" electrons then fall back to a lower energy state while emitting rays of light (photons). The emitted rays of light are controlled, for example in an oscillating reflective resonant cavity, and a portion of the amplified light is extracted as the laser output of the device.
Lasers are used in a wide variety of different applications. Low-power lasers are employed, for example, in short-range visible-light communication systems. Such communication systems are made possible because the laser light can be readily modulated, and it suffers far less angular divergence than ordinary light. Lasers may also be used for such diverse purposes as measurement of distances, measurement of velocities, and medical surgery. High-power lasers are used for heating and welding, and they are also used in weaponry systems.
Lasers generally operate in one of two modes: (1) a single frequency, narrow-band mode, or (2) a multifrequency broad-band mode. The mode of operation is determined by the type of laser medium used and the geometry of the resonating cavity. Where the cavity mode is narrow band, the laser emits laser light or photons having a single frequency. Due to doppler shift and other causes, this single frequency laser energy creates a very narrow band of frequencies at a receiving site. Often, the spectrum of such narrow band laser comprises three main frequency components, having wavelengths that may differ by only a fraction of an angstrom. Narrow band cavity mode lasers are useful in communication systems, and are exemplified by the HeNe laser. The present invention is not intended for use with such narrow band cavity mode operation.
In multifrequency broad-band operation, on the other hand, the laser emits a broad spectrum of laser frequencies within a gain band, having wavelengths that may vary over a wide range, such as 100 angstroms. Representative laser mediums that produce a broad band of laser frequencies include titanium doped sapphire or an organic dye. Broad band mode operation offers significant advantages over narrow band mode operation for many applications. For example, the laser mediums used with broad band operation are typically less expensive or may require less excitation energy than do narrow band laser mediums.
For many applications involving broad band mode operation, it is desireable to select a particular frequency (or narrow band of frequencies) from the broad band of frequencies available in the laser spectrum. (Note that it is also common to refer to the wavelength of the laser rather than the frequency of the laser, although one is directly relatable to the other in a given environment.) Such frequency or wavelength selection is referred to as "tuning" the laser. Many laser applications require that the laser be tuned rapidly, or switched from a first frequency (or first narrow band of frequencies) to a second frequency (or second narrow band of frequencies) in a very short time period.
Heretofore, the only techniques known for tuning a laser operating in the broad band mode of operation have involved mechanical devices that physically move or rotate an optical component, such as a grating or prism, relative to the optical axis of the resonant cavity. Disadvantageously, all such components have a certain inertial mass associated therewith that limits how fast they can be rotated or moved. Further, special drive mechanisms, such as motors, solenoids, galvanometers, and the like, are required to effectuate the movement of such components. These drive mechanisms all significantly add to the cost and complexity of the laser system, and further require constant maintenance and repair.
An alternative to tuning a single broad band laser in order to switch from one laser frequency to another is to switchably select the desired laser output from one of a plurality of narrow band cavity mode lasers. Disadvantageously, such a selection system not only requires multiple laser sources, each having its own resonating cavity, but also requires the optical switching components required for selecting the laser from one source and deselecting the laser from the other sources. These additional components further add to the complexity and cost of the system. Moreover, many of the selection components further involve moving mechanical elements, all of which further limit the speed with which the switching function can performed.
It is thus evident that there exists a need for a simple means of rapidly tuning a laser system that involves no moving mechanical components. It would also be desireable if such a tunable laser system utilized as few components as possible, i.e., if the selection could be performed by rapidly tuning a single broad band laser rather than switching between the output beams of a plurality of narrow band lasers. The present invention advantageously addresses these and other needs.